Metallurgy in Orthodontics: Understanding Molecular Structure and Properties

METALLURGY In Orthodontics
Presented by-
Vaibhav Ambashikar

CONTENTS
• Introduction
• Metallurgy
• History
• Periodic table
• Evolution of metals in Orthodontics
• Importance in Orthodontics
• Molecular structure and Interatomic bonding
3

• Lattice structure and Crystal arrangements.
• Lattice Defects
• Physical metallurgy
• Chemical metallurgy
• Properties of metal
4

• Annealing
• Corrosion
• Mechanical Properties
• Stainless Steel
• Conclusion
• References
5

Introduction
• Knowledge of fundamental principles governing the relationships between
compositions, structures and properties is central to an understanding
of orthodontic materials.
6

• Continuous search for ideal characteristics.
• Arch wires: Light continuous force over a long period of
time.
• Aim: Knowledge of basic metallurgy & orthodontic wire
characteristics.
7

What is a METAL?
• A material composed of one or more chemical elements that is opaque,
ductile, relatively malleable, a good conductor of electricity, a good thermal
conductor and usually lustrous when light is reflected on its polished
surface.
Anusavice: Phillips’ Science of Dental Materials 12th Ed
8

METALLURGY
Greek: Metallon- mine, Ergos- Worker in metal.
• The branch of science and technology concerned with the
properties of metals, their production and purification.
• Almost 80% of the known elements are metals.
Anusavice: Phillips’ Science of Dental Materials 12th Ed 9

HISTORY
• 6000 B.C. – Stone age man learned to fashion gold.
• 4200 B.C. – Copper used to make implements and weapons.
• 4000 B.C. – Silver nuggets used to make currencies and
ornaments.
10

• 2300 B.C. – Bronze Age begin.
• 1500 B.C. – Hittities discovered iron which gave them
advantage in wars.
11

• Around 1300 A.D. – the first forge was developed in
Spain called the Catala Forge.
12

Continuous shaft furnace
• Forerunner of the modern blast furnace.
• Developed in Germany at around 1323 A.D.
• The high carbon product of this furnace came to be known as cast iron.
• Broadened the use of iron castings.
13

Periodic Table
• Given by Russian chemist Dmitri Mendeleev in 1869.
• Based on atomic weight.
• Out of 118 elements on the periodic table, 88 of them are
metals.

Evolution of metals in Orthodontics
Early contributors:-
• In 1757, Etienne Bourdet advocated the Fauchard
method and recommended gold strips on the labial
and lingual surfaces of upper and lower teeth
respectively.
• In 1828, Spooner found the use of gold and silver
plates to exert a gentle and continuous pressure to
correct irregular teeth.
• In 1841, William Linott described a bite opening
appliance that consisted of a labial bar of gold or
silver around the front surface of teeth.
Graber,Vanarsdall,Vig: Orthodontics – Current Principles & Techniques 4th Ed 16

Enlightenment
• The early phase of the 20th century was dominated by
Edward Angle along with Norman Kingsley and Farrar.
• During this period, gold , platinum, silver and steel were
used.

Stagnation abounds
• From 1930s-1960s, the proliferation of materials did occur
with the death of Edward Angle.
• In early 1940s, Begg partner with Wilcock to make the
resilient orthodontic wire- Australian stainless steel.

Proliferation abound
• In 1962, Nitinol was discovered by Buehler and by 1986, two superelastic
alloys Japanese and Chinese NiTi was developed.
• In 1977, a beta phase titanium was developed which was stable at room
temperature.
• In 1994, three copper NiTi products were introduced that had chromium in
them as well.

Importance in Orthodontics.
• The correct use of appliances, the mechanical and physical properties
need to be known.
• The physical properties of the metal are a manifestation of their molecular
nature
20

Molecular Structure
• Atom- Smallest piece of an element that keeps its chemical properties.
(a-un, temno- to cut)

Interatomic bonding
• Primary Bonds
Also known as chemical bonds
Formation of primary bonds depends on the atomic structure and their ability
to form a stable configuration.
Three types:-
1. Ionic Bond
2. Covalent Bond
3. Metallic Bond
22

1. Ionic Bond
Transfer of valence electron from one atom to another in order to obtain a
stable compound.

2. Covalent bond
• A covalent bond, also called a molecular bond, is a chemical bond that
involves the sharing of electron pairs between atoms.

3. Metallic Bond
• Metallic bonding is a type of chemical bonding that arises
from the electrostatic attractive force between conduction
electrons (in the form of an electron cloud of delocalized
electrons) and positively charged metal ions.
25

• Secondary Bonds
They induce dipole forces that attract adjacent molecules or a part of a large
molecule.
Types:-
1. Van der Waals Forces
2. Hydrogen Bonds

1. Van der Waals Forces- They arise from dipole attractions.
27

2. Hydrogen Bonds
A hydrogen bond is a partially electrostatic attraction between
a hydrogen which is bound to a more electronegative atom
such as nitrogen, oxygen, or fluorine, and another adjacent
atom bearing a lone pair of electrons.

Based on the atomic arrangement, materials can be subdivided as-
1) Crystalline materials- The atoms are arranged in a regular, three
dimensional periodic pattern.
2) Non-crystalline materials- No periodicity is present in the arrangement of
the atoms, only short-range atomic order is present.
Brantley, Eliades Orthodontic Materials, 1st intl Ed
29

Brantley, Eliades Orthodontic Materials, 1st intl Ed 30

Lattice Structure
• A systematic arrangement of their atoms in the solid state.
• Each atom is surrounded by as many neighboring atoms as is
geometrically possible.
• A space lattice is defined as any arrangement of atoms in a space such
that every atom is situated similarly to every other atom.
Brantley, Eliades Orthodontic materials, 1st Intl Ed 31

Crystal systems and space lattice.

Types of arrangement
• Body Centered Cubic (B.C.C.): Has atoms at each corner of the cube of
a unit cell and one atom in the center. E.g., iron below 9120 C.

• Face Centered Cubic (F.C.C.): Has atoms at each corner of the cube and
one in the center of each of the six faces but none at the center. E.g. iron,
gold, silver, copper above 9120C.

• Hexagonal Close-Packed Structure (H.C.P.): Has atoms at each corner
of the hexagonal unit and one in each of the two-face units. E.g., zinc,
magnesium.
Brantley, Eliades Orthodontic materials, 1st Intl Ed
35

Lattice Defects
• Usually crystals have defects.
• 2 types Point defects:
Impurities:
Interstitial element.
Substitutional element.
Vacancies.
Line defects.
Brantley, Eliades Orthodontic materials, 1st Intl Ed
36

Point Defects
• Impurities:
Interstitial element:
• Smaller atoms penetrate the lattice structure.
• Often distort the metal.
37

38
Substitutional alloy
(e.g., Cu in Ni)
Interstitial alloy
(e.g., C in Fe)

• Vacancies:
• Empty atom sites.
• Distortion.
• Equilibrium defects.
• Diffusion of metals.
• Increases with temperature.
39

Line Defects
Dislocation
• Linear array of atoms.
• Coordination different from normal.
• Deformation of metals.
40

Slip Plane
• Plane along which an edge dislocation moves.
Anusavice: Phillips’ Science of Dental Materials 41

Physical Metallurgy
• Used to fabricate metal into useful products.
• Also by changing the shape of metal.
• Joining by brazing, welding and soldering.
• Finishing by electroplating and galvanizing.
42

Welding
• One of the most important advances in metallurgy.
• An operation by which two or more metal surfaces are
united, by means of heat &/or high compressive forces, in
such a way that there is continuity of the nature of the
material between these parts.
John Daskalogiannakis – Glossary of Orthodontic Terms
43

Soldering
• An operation in which metallic parts are joined by means of a filler metal
having a melting temperature below that of the parts to be joined which
wets the parent metals.
• < 450° C.
• > 450° C → Brazing.
• No participation by parent metals.
John Daskalogiannakis – Glossary of Orthodontic Terms 44

Extractive/Chemical Metallurgy
Mineral dressing:
• Done between mining the ore and extracting metal from it.
• Removes as much of waste material as possible from the ore.
• Done by grinding the ore and washing away of unwanted
constituents.
• Ore crushed and kept in a special oil/chemical.
• Agitated by air or gas bubbles.
• Specific minerals get attached to the bubbles.
• Removed as froth.
• Remaining minerals → gangue.Handbook of Metals, 1992 45

Roasting:
• Heated in the presence of air.
• Sulphur and other impurities combine with oxygen in the
air.
• Escape as gases.
• Only the metallic oxide is left.
Sintering:
• Heated to high temperature.
• Small particles join together to form coarse lumps.
• Due to surface tension.
Handbook of Metals, 1992
46

Smelting:
• Actual process of extracting metal.
• Ore is kept in a huge brick-lined furnace (blast furnace).
• Heated to a high temperature with coke and limestone in
it.
• Coke burns giving rise to CO after the reduction of iron.
• Impurities combine with the limestone to produce a liquid
collection of refuse (slag).
• Lighter than the metal and rises to the top.
• Removed through holes on the side of the furnace above
Handbook of Metals, 1992 47

Amalgamation:
• For Au/Ag.
• Finely ground particles are carried by solutions over plates
covered with Hg.
• Attracts the metal and combines with it (forming amalgam).
• Heated → Hg evaporates (recovered and recycled).
• Leaves a metallic sponge of pure Au/Ag.
Handbook of Metals, 1992 50

Properties of Metals
Physical properties
• Conduction of electricity.
• Conduction of heat.
• High reflectivity of light from polished surface (metallic
luster).
• Most deform, but do not shatter under pressure.
51

Various Theories
Free electron gas model
• Simplest theory on metallic bond.
• Positive ions immersed in a negatively charged gas or
sea of valence electrons.
• Gives the entire structure electrical neutrality.
• Valence electrons are free to move. 52

This model accounts for:
• Conduction of electricity → due to the mobility of free
valence electrons.
• Increase in electric resistance with increase in
temperature → vibratory motion of metal atoms
increases impeding the current flow.
• Ductility and malleability → layers of metal atoms can
be shifted upon each other without disrupting the
electron sea → plastic under pressure. 53

Band theory of solids
• All the electrons occupy the allowed energy levels.
• Closely spaced and practically continuous energy bands.
• Separated by gaps of varying magnitude.
• Electrons are not allowed.
55

Solidification of Metals
• Freezing of a pure metal in a clean & inert container.
• Temperature–time graph plotted.

Tf — temperature indicated by the plateau phase at BC 
freezing point/solidification temperature/melting point/fusion
temperature.

• Freezing/solidification – release of heat.
• Higher-energy liquid state  Lower-energy solid state.
• Energy difference = Latent heat of solidification.
• Tf to B'  Supercooling.
• Crystallization begins.
• Release of latent heat of fusion   in temperature to Tf.
• Remains until crystallization is complete at C.

DENDRITIC MICROSTRUCTURE
• A cast alloy microstructure.
• Highly elongated crystals.
• Branched morphology.
• Seen in base metal alloys.
• Thermal supercooling.
• Not desirable in cast dental alloys.
• Serve as sites for facile crack propagation.

EQUIAXED GRAIN MICROSTRUCTURE
• Three dimensions of each grain are similar.
• Seen in noble metal alloys.

GRAIN REFINEMENT
• Modern dental noble metal alloys.
• Incorporation of small amounts of iridium, ruthenium &
rhenium.
• Creation of equiaxed fine-grain microstructure.

Key Terms
Strain hardening/ work hardening:
•  in strength & hardness.
•  in ductility.
• Caused by plastic deformation.
Cold working:
• Plastically deforming a metal.
• Room temperature.
• Accompanied by strain hardening.

Annealing
• Controlled heating & cooling process.
• Desired properties in a metal.
• Intentions: To soften metals.
To  plastic deformation potential.
To stabilize shape.
To  machinability.
• The more severe the cold working, the more readily does
annealing occur.
65Anusavice: Phillips’ Science of Dental Materials 12th Ed

Annealing generally comprises of:
• Recovery.
• Recrystallisation.
• Grain growth.

Recovery
• Properties begin to disappear before any significant
visible changes are observed microscopically.
• Due to the thermal movement of dislocations.
• Align in a fashion to minimize total strain energy and to
remove strain from the system.

• A very slight decrease in the tensile strength.
• No change in ductility.
• A pronounced change in the recovery of electrical
conductivity.
• Tendency for warping disappears.
69

Recrystallization
• Occurs after some recovery after annealing.
• Radical change in microstructure.
• Old grains completely disappear.
• Replaced by a new set of strain free grains.

`
• Usually at grain boundaries, or where the lattice was most
severely bent on deformation.
• On completion → the material essentially attains its
original soft and ductile condition.

Grain growth
• Depends upon the number of crystals.
• The more severe the cold working, the greater the
number of such nuclei.
• Rather fine to fairly coarse.
• Grains begin to grow.

• Reach an ultimate coarse grain structure.
• Excessive annealing can lead to larger grains →
detrimental to the strength of a metal.
• The grain boundary acts as a barrier to the movement of
dislocations.
• ↓ Grain size → ↑ Concentrations of inhibited dislocations
per grain → ↑ Strength, hardness and proportional limit
during cold working.
• However, ↓ Ductility.

• a) After cold working.
• b) After recovery.
• c) After recrystallisation.
• d) After grain growth.
Anusavice: Phillips’ Science of Dental Materials 12th 74

Tensile Strength & Ductility v/s %Cold Work & Time of
Annealing
75

Summary
• Cold Working   Tensile Strength.
 Ductility.
• Recovery Slight changes.
• Recrystallization  Tensile Strength.
 Ductility.
• Grain Growth Slight Changes.
76

Corrosion
• Chemical or electrochemical process by which a solid,
usually a metal is attacked by an environmental agent,
resulting in partial or complete dissolution.
• Not merely a surface deposit (unlike tarnish).
• Chiefly 2 types:
1. Dry corrosion.
2. Wet/Electrochemical
corrosion.
77

Forms of Corrosion
1. Uniform attack.
2. Pitting corrosion.
3. Crevice corrosion/Gasket corrosion.
4. Galvanic corrosion.
5. Intergranular corrosion.
6. Fretting corrosion.
7. Microbiologically influenced corrosion.
8. Stress corrosion.
Eliades T, Athanosiou AE: In Vivo Aging of Orthodontic Alloys: Implications for Corrosion Potential, Nickel Release &
Biocompatibility;AO 2002 Vol 72 78

Uniform Attack
• Commonest type.
• The entire wire reacts with the environment.
• Hydroxides or organometallic compounds formed
• Detectable after a large amount of metal is dissolved.
Biocompatibility; AO 2002 Vol 72 Pg 222-37 80

Pitting Corrosion
• Identified in brackets and wires.
• Manufacturing defects - sites of easy attack.
• Maybe seen before insertion into oral cavity in as-received products.
Biocompatibility; AO 2002 Vol 72 Pg 222-37
81
Stainless Steel NiTi

82
Secondary electron images of the surfaces of an as-received
SS bracket (a) & a retrieved SS bracket(b).

Crevice Corrosion
• Application of non-metallic parts
on metal in a corrosive environment.
• E.g., elastomeric ligatures.
• Plaque build up depletion of O2 –
disturbance in the regeneration
of the passivating layer of Cr oxides.
• Crevice depth - 2-5 mm.
Eliades T, Athanosiou AE: In Vivo Aging of Orthodontic Alloys: Implications for Corrosion Potential, Nickel Release & Biocompatibility;
AO 2002 Vol 72 Pg 222-37 83

Galvanic Corrosion
• Combined process of oxidation & dissolution.
• Joining of two metals or even the same alloy subjected to
different treatments.
• Difference in the reactivity 
Galvanic cell
 
Less Reactive More Reactive
(Cathodic) (Anodic)
(Nobler)
AO 2002 Vol 72 Pg 222-37 84

Intergranular Corrosion
• Sensitization – alteration of microstructure.
• Due to precipitation of Cr carbide at the grain boundaries.
• Mainly affects the solubility of Cr carbide.
85

Microbiologically Influenced Corrosion
• Sulfate-reducing Bacteriodes corrodens, sulfur oxidizer
Thiobaccilum ferroxidans & acid-producing Streptococcus
mutans.
• Matasa – first to show evidence of microbial attack on
adhesives in orthodontics.
• Craters in the bracket base.
86

Microbial colonization of wires. 87

Stress Corrosion
• Similar to galvanic corrosion.
• Electrochemical potential difference created at specific sites.
• Bending of wires results in development of different degrees of tension and
compression locally.
• Stressed sites - act as anodes.
AO 2002 Vol 72 Pg 222-37
88

Corrosion Fatigue
• Tendency of a metal wire to fracture
under repeated cyclic stressing.
• Accelerated in a corrosive medium
such as saliva.
• Characterized by the smoothness of the fractured areas, which also
include a site of increased roughness & crystalline appearance.
Biocompatibility; AO 2002 Vol 72 Pg 222-37 89

Weld Decay
• Weld decay is a form of intergranular corrosion, usually
of stainless steels or certain nickel-base alloys, that
occurs as the result of sensitization in the heat-affected
zone during the welding operation.
90
Weld Decay
Weld Decay

• Due to precipitation of Cr carbide.
• In this case, the precipitation of chromium carbides is
induced by the welding operation when the heat affected
zone (HAZ) experiences a particular temperature range
(425oC-600oC).
91

Clinical Implication
To avoid weld decay:
•Use low carbon (e.g., 304L, 316L) grade of stainless
steels.
•Use stabilized grades alloyed with titanium (e.g., type 321)
• Use post-weld heat treatment.
92

Summary
• Metallurgy
• History
• Periodic table
• Evolution of metals in Orthodontics
• Importance in Orthodontics
• Molecular structure and Interatomic bonding
93

• Lattice structure and Crystal arrangements.
• Lattice Defects
• Physical metallurgy
• Chemical metallurgy
• Properties of metal
• Annealing
• Corrosion
94

REFERENCES
• Anusavice KJ. Phillips’ Science of Dental Materials. 12th
Ed.
• Brantley WA, Eliades T. Orthodontic Materials: Scientific
& Clinical Aspects.
• Handbook of metals, 1992
• Daskalogiannakis J. Glossary of Orthodontic Terms.
• Graber TM, Vanarsdall RL, Vig KWL. Orthodontics:
Current Principles & Techniques. 4th Ed.
95

• Proffit WR, Fields HW, Sarver DM. Contemporary
Orthodontics. 4th Ed.
• Eliades T, Athanosiou AE. In vivo aging of orthodontic
allots: Implications for corrosion potential, nickel release,
and biocompatibility. AO 2002; 72(3): 222-37.
96

Metallurgy in Orthodontics: Understanding Molecular Structure and Properties

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Metallurgy in Orthodontics: Understanding Molecular Structure and Properties

Similar to Metallurgy in Orthodontics: Understanding Molecular Structure and Properties (20)

Recently uploaded

Recently uploaded (20)

Metallurgy in Orthodontics: Understanding Molecular Structure and Properties

Editor's Notes